1. Field of the Invention
This invention generally relates to superconducting electrical machines and, more particularly, to apparatus and methods for generating a magnetic field therein.
2. Description of the Prior Art
The concept of a superconducting AC generator for utility power system applications has been of interest for a number of years, but it has not been until recently that definitive feasibility studies have been made. Although these studies are not yet complete, it appears that among the advantages to be accrued with such a machine will be reduced size and weight, higher efficiency, lower capital cost, and greater system stability. In addition, a major opportunity afforded by a superconducting generator is the potential for generation at full-line voltage, i.e. 230 kV, 500 kV, and higher. Operation at full-line voltage would totally remove the necessity for a step-up transformer and would result in a simplification of the central station, a potential increase in reliability, and savings in both capital and operating expenses.
As far as the Applicant has been able to determine, the first work reported in the open literature on superconducting alternators was conducted by Woodson, Stekly, Halas, Hatch, and Hoppie in 1966. This work is reported in IEEE Trans. Power Apparatus and Systems, PAS 85, 264, by H. H. Woodson, Z. J. J. Stekly, and E. Halas, 1966; and IEEE Trans. on Power Apparatus and Systems, PAS 85, 274, by Z. J. J. Stekly, H. H. Woodson, A. M. Hatch, L. O. Hoppie, and E. Halas, 1966. A 1964 AVCO research report 181 by Stekly and Woodson establishes an even earlier date. They constructed and successfully operated an alternator using a fixed superconducting field coil, and a rotating, 300.degree. K. armature with slipring connections.
The first rotating superconducting field coil was shown to be practical in 1971 by an MIT group. This work is reported in IEEE Trans. Power Apparatus and Systems, PAS 90, 611, by P. Thullen, J. C. Dudley, D. L. Greene, J. L. Smith, Jr., and H. H. Woodson, 1971. The machine was operated at 45 kVA as a synchronous condenser. These experiments demonstrated that an adequately stranded and transposed armature can take advantage of the high field produced by the rotor with acceptably low eddy-current and circulating current losses, and without needing iron to couple the flux from the rotor to the armature. This machine achieved 3.2 T in the rotating state.
A second machine, rated at 3 MVA, was built, tested, and operated on Nov. 25, 1975, for about 22 minutes as a synchronous condenser on the Cambridge grid. This machine had a rotating superconducting field coil inside a normal stationary armature.
In addition, Westinghouse has built and operated a 5 MVA superconducting generator, as well as a lightweight 12,000 rpm, 4 pole superconducting rotor for airborne applications. This work is reported in Proc. 1972 Appl. Superconductivity Conf., IEEE Pub. 72CHO682-5-TABSC, p. 151, C. J. Mole, H. E. Haller, and D. C. Litz; and Proc. 1974 Appl. Superconductivity Conf., IEEE Trans. on Magnetics, MAG 11, 640, by J. H. Parker, Jr., R. D. Blaugher, A. Patterson, P. D. Vecchio, and J. L. McCabria, 1975.
At the present time there are several problems in the design of an ordinary rotating superconductor. When the superconductor is the field coil of an electrical machine, there is the problem of conductor motion and training. If the field coil is stationary, the magnetic field at the presently comtemplated levels of 5--6 Tesla is sufficient to cause conductor motion. If the field coil is rotating, the centrifugal force is greater than the magnetic force and the two forces combine to cause motion of the superconducting strands in the rotor. Conductor motion, when it occurs, reduces the critical current density in the machine as well as increases the power loss. At cryogenic temperatures conductor motion represents a substantial loss in the refrigeration cycle. Further, if conductor motion is unrestrained, it can dissipate so much energy that the superconductor is driven normal.
The second problem in the design of an ordinary superconducting coil is the need for stabilization of the electrical machine from the possibility of magnetic, thermal, mechanical and electrical disturbances. Any one of these disturbances can severely degrade the performance of the superconductor. In addition, regenerative degradation can lead to catastrophic quench of superconductivity.
One of the crucial properties of superconductivity is the fact that a magnetic field is expelled from the bulk of a superconductor in a transition from the normal to the superconducting state. This effect is named the Meissner effect after its discoverer. However, contrary to the expectation from the Meissner effect, it has been shown that any field configuration from low to high field strength can be trapped in both a Type I and Type II superconductor. The fidelity of the trapped field to the original field has been shown to be quite high. Dipole, quadrupole, and sextupole magnetic fields have been permanently trapped traversely to the axis of solid, hollow, and split-hollow superconducting cylinders. This work is reported in IEEE Trans. on Magnetics, MAG 11, 548, by M. Rabinowitz, 1975; Nuovo Cimento Letters 7, 1, by M. Rabinowitz, E. L. Garwin, and D. J. Frankel, 1973; and Appl. Phys. Letters 22, 599, by E. L. Garwin, M. Rabinowitz, and D. J. Frankel, 1973.
Objects and Summary of the Invention
It is an object of the present invention to reduce the size and weight, to increase the efficiency, to lower the capital cost, and to provide greater stability in a superconducting electrical machine.
A further object of the present invention is to increase the magnetic flux density (B) in a superconducting electrical machine and thereby increase the output power of the machine. The power density of either an electrical generator or a motor is proportional to the square of the average flux density (B) at the armature.
An additional object of the present invention is to eliminate conductor motion and training in the rotor of a superconducting electrical generator.
Still a further object of the present invention is to incorporate A-15, beta-tungsten structure, superconductors into electrical rotating machine. These materials not only permit operation at much higher levels of magnetic flux density (B) but also permit more stable operation at the same field values as wound coil rotors using lower critical parameter materials. It should be understood that these A-15 materials are so brittle that they are not easily fabricated into wire and consequently have not heretofore been used in wirewound rotors.
A further object of the present invention is to trap a magnetic field for use in a superconducting electrical machine by using the armature coils of that machine.
Another object of the present invention is to eliminate all electrical leads to the rotor of a superconducting generator. These leads heretofore have been lossy since they are refrigerated and have required complex superconducting-to-normal transitions, and slipring assemblies.
Additional objects and features of the invention will appear from the following description in which the preferred embodiments have been set forth in detail in conjunction with the accompanying drawings.